An auxiliary power unit (APU) on an aircraft may be a gas turbine engine engaged to produce electrical and pneumatic power. The power demand loading the APU may be variable during APU operation. Typically, in order to maintain constant generator frequency the APUs operate at constant speed. The operating speed is chosen such as to optimize the engine performance at maximum loading conditions and compromising the performance at partial load conditions. In modern applications, this restriction is removed and the APU can produce electrical power with variable frequency. However, APUs operating at variable frequencies may result in suboptimal engine performance at some power conditions.